Smithsonian miscellaneous collections . of the lateral coelomic compart-ment of the same segment with the exterior (fig. 2D, Nph). Theexcretory head glands of Crustacea are organs of the same type ofstructure. An onychophoran or chilopod gonoduct, as shown by itsdevelopment in Pcripatus and Scolopendra (fig. 3 B, D), representsan entire coelomic sac together with its exit tube. Hence, if anephridium of the onychophoran type is once formed in a segment,it cannot be converted into a genital duct. The onychophoran nephridia,on the other hand, might be regarded as remnants of segmental gono-ducts,
Smithsonian miscellaneous collections . of the lateral coelomic compart-ment of the same segment with the exterior (fig. 2D, Nph). Theexcretory head glands of Crustacea are organs of the same type ofstructure. An onychophoran or chilopod gonoduct, as shown by itsdevelopment in Pcripatus and Scolopendra (fig. 3 B, D), representsan entire coelomic sac together with its exit tube. Hence, if anephridium of the onychophoran type is once formed in a segment,it cannot be converted into a genital duct. The onychophoran nephridia,on the other hand, might be regarded as remnants of segmental gono-ducts, if it be assumed that each coelomic sac primitively contained a NO. 14 INSECT ABDOMEN—SNODGRASS germarial area in its wall, of which there is no specific evidence. Theerroneous idea that arthropod gonoducts are modified nephridia isprobably in part a result of the different significance of the term gonad as used by students of Annelida and Arthropoda, but it islargely based on a failure to recognize the essential difference between. Gdc c Amp 2Gp NC 2Gp Fig. 3.—Development of the genital ducts in Onycjiophora and Chilopoda.(A, B from Kennel, 1888; C from Evans, 1902; D from Heymons, 1901.) A, cross-section of a pregenital segment of Pcripatus, diagrammatic, showingsubdivision of the coelomic sacs and beginning of formation of coelomoducts{c, d). B, same through genital segment, showing formation of genital ductsfrom coelomic sacs and coelomoducts. C, section of genital segment of Eoperi-patus wcldoni with ectodermal gonoductus communis (Gdc) giving outlet tothe lateral ducts. D, diagrammatic cross-section of genital segment of Scolo-pcndra with coelomic sacs that will become genital ducts. a, dorsal compartment of coelomic sac; AlCnl, alimentary canal; Amp,ampulla of coelomic sac in rudimentary gonopod; b, lateral compartment ofcoelomic sac; c, ventral diverticulum of coelomic sac; Coel, coelomic sac; d,ectodermal coelomoduct; DV, dorsal blood vessel; Gdc, gonoductus commu
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Keywords: ., bookcentury1800, bookdecade1860, bookidsmiths, booksubjectscience
